1. Structure of the Tetrahymena Ribozyme
Feng Guo, Anne R. Gooding, Thomas R. Cech 
Molecular Cell 
Volume 16, Issue 3, Pages (November 2004)
DOI: /j.molcel

2. Figure 1
The Group I Intron Splicing Mechanism and Its Related Enzymatic Activity
Self-splicing begins with the binding of an exogenous molecule of guanosine or one of its 5′-phosphorylated derivatives (GOH, where the subscript represents the free 3′-OH group). The G-site is located in paired region P7 of all group I introns. The reaction proceeds through three chemical steps with release of a short oligo with the exogenous G linked at its 5′ end (Chem.3). Wavy lines represent the 5′ and 3′ exons and thick lines stand for the intron. Watson-Crick and G:U wobble base pairs are indicated by thin bars and dots, respectively. The enzymatic form of the ribozyme (viii) closely resembles form vi, which is the product of the exon ligation step of splicing and the precursor for intron circularization. The crystallized ribozyme (x) is the enzymatic form in the absence of its substrate. The cleavage activity (from viii to ix) of this ribozyme under crystallization conditions is shown in the insert. The substrate (S) includes nucleotides in the paired regions 1, 2, and 2.1. The cleavage product (P) terminates at the 5′ splice site. M is a synthetic marker chemically identical to the product.
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3. Figure 2
Stereoview Showing a Portion of the 2Fo − Fc Composite Omit Map
Electron density is contoured at 1.0 σ cutoff.
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4. Figure 3 Structure of the Tetrahymena Ribozyme
(A) The secondary structure of the crystallized ribozyme. The P4-P6 and P3-P9 domains are indicated by blue and green backgrounds, respectively. The G binding site in base-paired region 7 (P7) is highlighted by a red background and 3′-terminal ωG in gold. The positions of the five stabilizing mutations are circled. The residues mutated to facilitate crystallization (Golden et al., 1998) are bracketed. Magenta lines represent interdomain interactions that are observed in at least three of the molecules in the asymmetric unit.
(B) RIBBONS diagram of the crystal structure of the ribozyme B molecule, with the same color scheme as in (A).
(C) Stereo C1′ traces of the four independent molecules in the crystal, superimposed with each other. Molecules A, B, C, and D are in red, black, green, and blue, respectively.
(D) A base triple interaction between the J3/4 (green) and P6 (blue) regions in molecule B. The secondary structure where this triple (highlighted by an oval) resides is shown on the left.
Molecular Cell  , DOI: ( /j.molcel )

5. Figure 4 Structure of the Guanosine Binding Site
(A) Three-dimensional structure of the G-site in molecule B. The 3′-terminal ωG residue, which is the active site nucleophile, forms a coplanar base triple with the base pair G264-C311; this triple is sandwiched by three other triples colored as red, blue, and magenta, respectively.
(B) G-site secondary structure deduced from the crystal structure, using the same color scheme as in (A). Base triple interactions are shown as dashed lines.
(C) The base triple interactions between the ωG residue and the G264-C311 base pair.
(D) The base triple of A263, C262, and G312 above the ωG.
(E) The base triple of A261, A265, and U310 below the ωG.
Molecular Cell  , DOI: ( /j.molcel )

6. Figure 5 Stereo Representation of the Eu and Ir Sites in the Ribozyme
The model-phased anomalous Fourier's (Eu as cyan and Ir as magenta) are contoured at 6 σ cutoff. All four ribozyme molecules have similar Eu and Ir binding sites. The B molecule is shown in the figure. The ωG is drawn as gold and the G binding site as red. These heavy atom sites are assumed to be magnesium ions in the final refined native structure, with slight modifications to compensate for the nonisomorphism between crystals and to optimize the local coordination geometry.
Molecular Cell  , DOI: ( /j.molcel )

7. Figure 6 A Metal Ion at the Active Site
(A) The active site structure in the B molecule. The magnesium ion deduced from a strong Eu site is shown as a blue ball. The Eu anomalous electron density is contoured at 6 σ. The oxygen atoms on the three phosphates close to the magnesium ion are indicated as red.
(B) Proposed catalytic role of the magnesium ion prebound at the active site. In the presence of the substrate (represented by the structure of the cleavage site phosphate in the transition state in gray), the interaction between the metal ion and the 2′-OH of ωG orients the 3′-OH for nucleophilic attack (arrow). Dotted P…O bond, a bond half-broken in the transition state.
Molecular Cell  , DOI: ( /j.molcel )